As disclosed by Japan Utility Model Laid-Open No. HEISEI 2-14726 or the like, it is well known, in a structure of a push button switch, that the conventional film covering key tops composes of a sheet of film printing a display portion and a thermoplastic substance of the key top body produced by a molding, wherein a curvature portion having the same profile as the upper face of the key top body is disposed on the film and the upper face of the key top body is contained in the curvature portion so as to integrally unit the film with the key top body.
Further, as disclosed by the inspection of Japan Patent Laid-Open No. SHOWA 54-154461 and Japan Patent Publication No. HEISEI 7-54656, a manufacturing technology of key tops in which thermoplastic resin is buried into a resin film by injection molding, is also well known.
A structure of the push button switch as disclosed by Japan Utility Model Laid-Open No. HEISEI 2-14726, is characterized in its waterproofness, however it does not describe about chattering error key activation due to an insufficient glueing between the film and the key body which causes a problem for practical operation, an occurrence of wrong inputting due to operating together with adjacent keys, or materials and characteristics necessary for the film in accordance with a profile of the key body.
In the key top which is manufactured through a step of buring thermoplastic resin into resin film by means of injection molding as disclosed in Japan Patent Laid-Open No. SHOWA 54-154461 and Japan Patent Publication No. HEISEI 7-54656, resin is used as the film material. Therefore it has a disadvantage that the formation of a cavity with a sufficient depth is restricted, for example, in respect of the height and the shape of key which is important elements of a key top for portable telephone, the design of key shape for portable telephone is greatly limited, as clearly noted in Japan Patent Publication No. HEISEI 7-54656.
Further, as described in paragraph 0031! of Japan Patent Publication No. HEISEI 7-54656, the design of the case of a portable telephone is restricted in the height of key top, so said design cannot be applied for three dimensional configuration, as represented in perspective views from lateral face and bottom face sides in FIG. 4. To overcome the problem mentioned above, it is considered to arrange the film along the curved surface of a case face a as illustrated by FIG. 5, nevertheless the process has as well a limitation for the formation, because the film is formed with resin. As undue stress is applied to the film, the distortion of the keys is caused during the mounting. For the above reasons, an input portion of a portable telephone set is limited so as to be almost flat, so the whole design of the telephone set is resultingly restricted.
Furthermore, for instance, in a sheet type key top used for a portable telephone set, there is sometimes the case where the portable telephone set is left behind in a car, therein under the test of leaving it in a temperature atmosphere of 85.degree. C. which is generally a standard test for car equipments, the sheet type key top is distorted by residual stress in the resin film to return to the state before molding, thereby the key top cannot accomplish sometimes the function of itself.
Still further, in the case where key top elements molded using an elastic film such as an elastomer film or the like are applied as an input portion, the key tops are scratched by fingernails during key operation, or when the user puts the set into his/her bag or the like, the surface of key top may be easily damaged because the elastic film lacks sufficient hardness, and may not look nice outer appearance. Furthermore if thermoplastic substance contained in the curvature portion of the film is composed of resin, the key top is enough hard, and is in the direction to be hard of said tendency. The key top feeling during the input operation to appliances is limited by the resin feeling, as the result, a soft feeling such as a rubber switch can not be achieved.
In an integral molding, when an adhesive is applied onto the film, when a display portion of characters or the like is printed, or when an adhesive layer is provided after printing the display portion on the film, there has been a known technology for preventing heat from spreading towards the printed portion during injection molding, but since the adhesive is applied during or after molding, it is required that the adhesive is unhardening, thus the elongation of the printed portion is caused in accordance with the stretching of film which is heated during injection molding. Accordingly, although the printed portion itself is relieved of heat, the stretching of film can not be ristricted, and the distortion or the elongation of printed display portion cannot be avoided. Therefore the restrictions of the conditions of injection molding and the profile of key top are unavoidable.
When two or more key tops are composed of resin film, or when the film end is fixed (adhered, bonded or held) with other member, suppose that one of keys is depressed on key operation. At that time, adjacent keys may be also activated together through being pulled in the direction of film pushing. At the same time, input operations of keys are acted, and there has been a problem of error activation of appliances or unachieving of a desired operation or the like.